To avoid acceleration due to gravity driving downhill for miles, road or rail vehicles cannot use only their normal drum or disc brakes without great risks. Drum or disc brake capacities for heat dissipation are less than the quantity of heat that has to be dissipated during such downhill driving. Downhill braking using the transmission and the engine can be sufficient for vehicles with a very high power ratio, that is, 50 to 100 HP per ton. But for heavy vehicles with 8 to 10 HP per ton power rate, it is better to use a retarder which has to be installed between the transmission and the driving wheels.
Different types of retarders are known, most of them being electromagnetic or hydrodynamic. They are rather heavy and costly. The energy is converted into heat around the transmission shaft, and their braking torque at low speeds is very small.
The present invention relates to a hydrostatic retarder having a hydrostatic oil pump, preferably assembled on the input shaft of the differential, this pump forcing oil through a spill valve with pressure control, the oil passing from the spill valve to a cooling tank. The braking energy with this device is not converted into heat in the rotating parts, but in the spill valve, the heat being dissipated in a cooling tank. High braking torque at very low speeds can be generated, and this allows the use of this retarder as normal drum or disc brakes, in the case of failure of the normal brakes.
The conduits joining the pump, the spill valve and the cooling tank are large enough to minimize energy losses when the spill valve is completely open. For fast vehicles, it is preferably to have a clutch with synchronism between the input shaft of the differential and the oil pump. The synchro means need only transmit the torque necessary to put the pump and oil in motion, the spill valve being completely open and the oil being cold. Action on the spill valve control gives for each position of the control an oil pressure between the pump and the spill valve which is almost independent of the speed of rotation of the pump. This means that a braking torque at the input shaft of the differential for each position of the spill valve control will arise, which torque is independent of the speed of the vehicle.